Medical Errors and Adverse Events

Medical errors and adverse events are the central subject matter of patient safety: the unintended failures and harms that arise during health care. A medical error is a failure of a planned action to be completed as intended, or the use of a wrong plan to achieve an aim, while an adverse event is harm to a patient caused by medical care rather than by the underlying disease. The two concepts overlap but are not identical, because not every error reaches the patient or causes harm, and not every adverse event results from an error.

Definition

Medical errors and adverse events comprise the failures of health care processes (errors and near misses) and the resulting harms to patients (adverse events), together with the frameworks used to define, classify, detect, and measure them.

Scope

This area orients the reader to how harm in health care is conceptualized, named, detected, and studied. It introduces the vocabulary of errors, near misses, and adverse events; the distinction between preventable and non-preventable harm; the systems view of error causation; and the major problem areas of diagnostic and medication-related harm. It treats these as reference and educational topics within quality and patient safety, not as operational safety procedures or clinical guidance.

Sub-topics

• Adverse Event Detection Methods
• Diagnostic Error
• Error Taxonomy and Definitions
• Medication Errors

Core questions

• What distinguishes an error, a near miss, and an adverse event, and which adverse events are preventable?
• How is harm from health care detected and measured, and why do methods disagree on its frequency?
• Why does a systems view attribute most errors to conditions and processes rather than to individual blame?
• Which clinical domains, such as diagnosis and medication use, account for the largest share of preventable harm?

Key concepts

• Error versus adverse event
• Near miss (close call)
• Preventable versus non-preventable harm
• Active failures and latent conditions
• International Classification for Patient Safety
• Incidence of in-hospital harm

Key theories

Systems model of error (Swiss cheese)

Reason's framework distinguishes active failures by frontline operators from latent conditions built into systems, and argues that harm occurs when weaknesses in successive defensive layers momentarily align; it underpins the move from individual blame toward system redesign in patient safety.

Mechanisms

The systems view holds that adverse events rarely stem from a single careless act; instead, latent conditions in the design of work, equipment, staffing, and organization create the setting in which active failures occur, and harm follows when multiple defensive layers fail together. Standardized terminology, such as the World Health Organization's International Classification for Patient Safety, provides shared definitions that allow incidents, contributing factors, and degrees of harm to be described consistently across settings.

Clinical relevance

Understanding how errors and adverse events are defined and detected is foundational to reading patient safety literature and to interpreting reported rates of harm. This area describes how harm is conceptualized and studied at the level of systems and populations; it is not a source of individual diagnostic or treatment recommendations.

Epidemiology

Landmark record-review studies established that adverse events affect a substantial minority of hospital admissions and that a large share are judged preventable. The Harvard Medical Practice Study reported adverse events in roughly 3.7 percent of hospitalizations, and later trigger-tool and record-review work continued to document high rates of harm, with some studies finding little improvement over time. Reported frequencies vary widely with the detection method and the definitions used, which is itself a central methodological theme of the field.

History

Although iatrogenic harm has long been recognized, the field crystallized when the Harvard Medical Practice Study (1991) quantified adverse events in hospitalized patients and when the Institute of Medicine's 2000 report To Err Is Human brought the scale of preventable harm to public and policy attention. James Reason's systems model reframed error as an organizational rather than purely individual phenomenon, and subsequent international efforts produced shared classifications and measurement tools.

Debates

How frequent is preventable harm, and is it declining?

Estimates of adverse event rates differ markedly depending on whether harm is identified by voluntary reporting, record review, or trigger tools, and some longitudinal studies have found little reduction over time despite widespread safety initiatives, leaving the true trajectory contested.

Key figures

• James Reason
• Lucian Leape
• Troyen Brennan
• William Runciman
• Christopher Landrigan

Seminal works

• kohn-corrigan-2000
• reason-2000
• brennan-1991
• runciman-2009

Frequently asked questions

Is every adverse event the result of a medical error?

No. An adverse event is harm caused by medical care, but it may occur even when care was delivered correctly; the subset of adverse events judged to be caused by error is termed preventable harm.

What is a near miss?

A near miss, or close call, is an error or unsafe situation that did not reach the patient or did not cause harm, often because it was intercepted; near misses are studied because they reveal system weaknesses without the harm.